1. Field of the Invention
This invention relates to a parallel LD/PD module which simultaneously transmits and receives a plurality of optical signals via a plurality of optical fibers. A parallel communication system make use of a ribbonfiber containing four, eight, sixteen or, in general, 2m (m; integer) parallel element fibers for transmitting a plurality of optical signals. The number M (=2m) of channels is equal to the number of element fibers. A diameter of a cladding of a single-mode fiber is 125 μm. A standardized ribbonfiber includes parallel element fibers at a pitch of 250 μm. A ribbonfiber has 2m=M (4, 8, 16, 32, . . . ) element fibers at a 250 μm pitch in parallel.
Photodiode chips for optical communications are a square of about 500 μm×500 μm. Laser diode chips are a square of a side larger than 300 μm. An M (=2m) channel LD/PD module should be equipped with M laser diodes and M photodiodes. If M parallel lightpaths are formed at a 250 μm pitch, which is equal to the pitch of the ribbonfiber, in an LD module, laser diodes cannot be mounted at ends of parallel light paths of 250 μm pitch.
This application claims the priority of Japanese Patent Application No. 2002-032453 filed on Feb. 8, 2002, which is incorporated herein by reference.
2. Description of Related Art
{circle around (1)} Masato Shishikura, Kazuyuki Nagatuma, Tatsumi Ido, Masahide Tokuda, Koji Nakahara, Etsuko Nomoto, Tsurugi Sudoh & Hirohisa Sano, “10 Gbps×4 channel parallel LD module”, Proceeding of the 2001 Communications Society Conference of IEICE General Conference, C-3-50, p160 pointed out problems of large crosstalk, interference, heating and fluctuation of properties induced among laser diodes which would be arranged at a narrow pitch in a parallel LD module. {circle around (1)} proposed an improvement of a parallel LD module having width enlarging lightwaveguides for joining element fibers of a ribbonfiber at a front end and joining laser diodes at a rear end.
FIG. 16 shows a perspective view of a module proposed by {circle around (1)}. The module has a silicon bench (or base) 222 having a Si (100) plane surface. The silicon bench has SiO2 type lightwaveguides on the top surface. The lightwaveguides consist of a lower cladding layer, an intermediate core and an upper cladding layer. The cladding layers are made of silicon dioxide (SiO2). The core is a germanium (Ge) doped silicon dioxide (SiO2) square-sectioned line. The proposed one is a four channel LD module which has four lightwaveguides (cores) A, B, C and D. A unit distance of repetitions of guides or fibers is called a “pitch”, or a “spatial period” or simply a “period”. The pitch of the cores of the lightwaveguides is 250 μm at an initial part (front ends). The width between neighboring cores expands at an intermediate region. The pitch of the cores is 1000 μm (1 mm) at a rear part (back ends). Four laser diodes LDa, LDb, LDc and LDd are mounted with a 1000 μm pitch at back ends of the cores on a rear region of the silicon bench 222.
Aforecited {circle around (1)} enlarges the pitch of lightpaths from 250 μm, which is a standardized pitch of typical flat multifibers or tapefibers, to 1000 μm (1 mm) for admitting laser diodes to occupy enough areas. The intermediate width enlarging region widens the lightpath pitch to four times as wide as the original value. {circle around (1)} describes that the crosstalk between neighboring laser diodes (LDa and LDb; LDb and LDc; LDc and LDd) is as small as −40 dB at a signal frequency of 10 GHz. The large spacing is required for suppressing the crosstalk between signal generating laser diodes by {circle around (1)}. The intermediate width expanding region is necessary for allowing the lightwaveguides to mount laser diodes at final ends and reducing mutual crosstalk among the LDs. Smooth pitch enlargement requires a long silicon bench of a length between 15 mm and 20 mm. Long lightwaveguides incurs a large silicon bench and a bulky LD module.
Futuristic high speed parallel transmission requires such a configuration of arranging a set of individual laser diodes (LDs) at ends of parallel lightwaveguides. Allocation of individual laser diode chips allows the module to select an p-type substrate laser diode or an n-type substrate laser diode freely. A variety of oscillation frequencies can be assigned to the individual laser diodes (LDs). Adoption of the individual laser diodes enhances the freedom of designing.
The known reference {circle around (1)} shows only a transmuting device without a receiving device. A receiving device containing plural photodiodes would be prepared as an independent device isolated from the transmitting device. The separated receiving device would require another set of element fibers for guiding receiving optical signal beams. Two (transmitting and receiving) signals of the same wavelength propagate, for example, 1.3 μm in two independent sets of ribbonfibers. Since the transmitting device is independent from the receiving device and two independent devices utilize two independent sets of ribbonfibers, no crosstalk occurs between the receiving device and the transmitting device in {circle around (1)}. Namely the known reference {circle around (1)} is a binary fiber type which requires one ribbonfiber for a transmitting device and another ribbonfiber for a receiving device. An M-channel bidirectional communications requires 2M element fibers instead of M element fibers. Low crosstalk is an advantage of {circle around (1)}. But the binary fiber type {circle around (1)} would be a large-sized, high-cost module which requires two sets of ribbonfibers, an independent receiving device and an independent transmitting device.
If a four channel parallel communications system (M=4) were constructed in accordance with the teaching of the binary fiber type {circle around (1)}, a module would require an independent four channel receiving device, an independent transmitting device, and eight parallel element fibers (two fibers per channel; 4×2=8). Installation of binary sets of fibers would raise the cost of constructing the binary system.
A preferable module is a module which enables an optical communication network to bring four channel signals in both directions by four fibers instead of eight fibers. A purpose of the present invention is to provide a multichannel LD/PD module which enables M optical fibers to carry M channel signals simultaneously in both directions. Namely the number of the fibers is equal to the number of the channels. Another purpose of the present invention is to provide a multichannel LD/PD module of low-cost and small-size. A further purpose of the present invention is to provide a multichannel LD/PD module which can alleviate optical, electrical and electromagnetic crosstalk between a transmitting portion and a receiving portion.
A single fiber type, single-channel LD/PD module which horizontally disposes a wavelength selective filter, a laser diode (LD) and a photodiode (PD) on SiO2 type lightwaveguide layer formed on a silicon bench has been known. For example, {circle around (2)} Japanese Patent Laying Open No.11-68705, “TWO-WAY WDM OPTICAL TRANSMISSION RECEPTION MODULE” proposed a single-channel LD/PD module which has a silicon bench, a y-branched SiO2 lightwaveguide formed on the silicon bench, a laser diode (1.3 μm) deposited at an upper left end of “y”, a photodiode (1.55 μm) deposited at a bottom end of “y”, an end of a fiber fitted at a upper right end of “y” and positions a WDM (wavelength division multiplexer) at the branch for allowing 1.55 μm to pass and reflecting 1.3 μm at a 120 degree reflection angle. On the silicon bench, the 1.3 μm LD beam depicts a V-shaped locus and the 1.55 μm PD beam a/-shaped locus. The known reference {circle around (2)} contrives to reduce electrical crosstalk by positioning the LD and the PD in reverse directions regarding the WDM. Since {circle around (2)} is a module on an ONU (optical network unit; subscriber), a single-channel is sufficient.
An ONU is satisfied with a module having a single LD (1.3 μm) and a single PD (1.55 μm). The relation of the wavelengths is reversed for an ONU and a station. The central station should be equipped with a station module having an LD which emits 1.55 μm and a PD which senses 1.3 μm. The central station may utilize single-channel modules similar to the module of an ONU. The central station should have N single-channel modules for exchange signals with N ONUs. N, which is the number of ONUs, is a very large number. Installation of N single-channel modules would occupy a vast volume in the central station.
Instead of single-channel modules, multi-channel modules are favorable for a central station for alleviating the space of installing communication modules at the station. Most of the volume of a module is consumed by benches, packages and cases. PDs, LDs and lightpaths are small elements. A multichannel module, for example, four channel, eight channel, sixteen channel, or thirty-two channel module would be made to be a small size nearly equal to a single-channel one. A demand of multichannel modules for station modules occurs. An extension of the teaching of the single-channel {circle around (2)} that couples PDs and LDs to fibers by horizontal, planar y-branches may be a candidate of multichannel modules. The virtual extension model, which may be called a planar type which connects individual sets of a laser diode and a photodiode by a y-branch on a silicon bench, would consume a huge space for a plurality of y-branches on the silicon bench. The virtual planar M-channel module would be similar to a series of horizontally aligning M single-channel modules. Such a planar type is insignificant for the purpose of reducing size and cost of station communication modules.
If photodiodes (PDs) were provided near laser diodes (LDs) for the sake of reducing the module size, LD/PD access would raise optical crosstalk and electrical crosstalk between the LDs and the PDs. Large optical, electrical crosstalk prohibits the LD modules from transmitting optical signals simultaneously in bilateral directions. An enough distance should be maintained between PDs and LDs in the longitudinal direction and in the lateral direction for suppressing the LD/PD crosstalk.
What is the reason that conventional single-channel bidirectional LD/PD modules should require such a wide two-dimensional extension of, for example, {circle around (2)} Japanese Patent Laying Open No.11-68705 which unifies and divides a transmitting beam and a receiving beam by a y-branch horizontally formed on a silicon bench ? The reason causing such a wide extension is that conventional bidirectional modules two-dimensionally divide and unify two different wavelengths (e.g., 1.3 μm and 1.55 μm) on a common level of the silicon bench. Planar, two-dimensional unification or division of two beams causes such a y-branch which forces a silicon bench to consume a wide area.
Area-consumptive y-branches contradict the requirement of producing small-sized LD/PD modules. {circle around (2)}, which is a single-channel LD/PD module which has a single LD and a single PD, may submit to enlargement of size induced by the planar y-branch. Multichannel transmission will urge LD/PD modules to reduce the size.
A single bidirectional LD/PD module has an intrinsic weak point of electrical crosstalk and optical crosstalk between a laser diode and a photodiode. Access of PD/LD increases the crosstalk. Large crosstalk disturbs optical communications. A photodiode should be far separate from a laser diode for reducing crosstalk in an LD/PD module. For the purpose, the known reference {circle around (2)} positions the photodiode at a bottom end point of “y” far distanced from the laser diodes laid at a top left end of “y”. The separation increases the length of the silicon bench. Allotment of a wide planar distance between a laser diode and a photodiode contradicts the purpose of reducing the size of a module.